How can I place vertices on the very center of the uv image?
There might be a better way, but here's a quick and dirty method:


In the UV/Image editor, select all vertices.
In the UV/Image editor's N-panel, set the UV vertex position to X: 500; Y: 500.
With all vertices still selected, scale 50% in the x-direction by tapping S, X, then typing ".5", Return.
In the N-panel, set the UV vertex position to X: 250.